Triphosphor or three primary color phosphor is prepared by blending three color phosphors, including the red phosphor giving the main emission peak at the wavelength of 611 nm, the green phosphor at 544 nm, and the blue phosphor at 450 nm when excited by ultraviolet light of the wavelength of 253.7 nm, the brightness being mainly contributed by the green. In 1976, the Philips Electronics of Netherlands developed the magnesium polyaluminate based green phosphor coactivated by Ce and Tb, making an excellent contribution to the development of triphosphor fluorescent lamps (EP 1458700). However, the very high firing temperature in excess of 1500.degree. C. and prolonged heating as required by such green phosphor resulted in hard caking of the calcined mass that was difficult to undergo post-firing treatment. Low material yield and short service life of the refractories used led to a high production cost. Therefore, active studies and increasing commercial applications are being directed to the phosphate based green phosphor coactivated by Ce and Tb.
Phosphate based green phosphors were reported fairly early. In 1968, R. C. Ropp of Westinghouse Electric Corp. studied phosphors made by using LaPO.sub.4, YPO.sub.4, and GdPO.sub.4 as matrixes and Eu.sup.3+, Ce.sup.3+, Tb.sup.3+, Sm.sup.3+, Tm.sup.3+, and Dy.sup.3+ as activators [J. Electrochem. Soc., 115, 841 (1968)]. In 1971, M. V. Hoffman of General Electric studied a phosphor of composition (La.sub.1-x-y Ce.sub.x Tb.sub.y) PO.sub.4 [J. Electrochem. Soc., 118, 1508 (1971)]. In 1979, Mitsubishi Electric Corp. of Japan reported a green phosphor of composition (La.sub.1-x-y-p-q Gd.sub.x Y.sub.y Ce.sub.p Tb.sub.q) PO.sub.4 [JP 54-56086] and 0.15 was considered as the optimized value of p in the composition formula, any further increase in p would bring about a decrease of brightness. All phosphate based green phosphors reported previously suffered from low brightness and could hardly be used in commercial production.
In 1982, Nichia Chemical Industries Ltd of Japan reported on the phosphor of composition (Ce.sub.1-x-y La.sub.x Tb.sub.y) PO.sub.4 [JP57-23674], where 0.1&lt;(x+y)&lt;0.4, 0.05&lt;y&lt;0.3. The phosphor was prepared by the well known dry blending method or the wet precipitation method. In that invention, phosphor brightness was increased by displacing small amount of Ce by La and high firing temperature &gt;1500.degree. C. was also obviated. But the Ce.sup.3+ concentration in the phosphor was relatively high (1-x-y=0.7 was considered as optimized) and Ce.sup.3+ had a strong tendency of being oxidized making the phosphor less stable, less durable, easier to decay in brightness and more vulnerable to ultraviolet emission. When dry blending process was used for its preparation, the phosphor was found to agglomerate and stick to the wall since (NH.sub.4).sub.2 HPO.sub.4 was hygroscopic. This made it difficult to obtain a homogenous mixture, resulting in local excess or insufficiency of P/RE (phosphorus and rare earth ratio). It was therefore difficult to obtain high performance phosphor. When using the wet precipitation process, the phosphate obtained was small in particle size, making its washing and filtration difficult and resulting in formation of cakes which needs disintegration. Moreover, the particle size distribution of the green phosphor made was rather wide due to the presence of a large amount of fine particles. The quality of the coat film was, therefore, not satisfactory and its luminous efficiency was low.